In June, Olympic gold medal-winning decathlete Caitlyn Jenner—formerly known as Bruce Jenner—came out as transgender. It was a watershed moment in a year where transgender issues have been consistently making headlines. Now, Jenner is considered one of the most famous openly transgender people in the world. But before she became a transgender icon, before she was on Keeping Up with the Kardashians, she was an athlete. And her public transition arguably makes her the most famous transgender athlete in the world. (In fact, her heartfelt speech was one of the 10 Amazing Things That Happened at the ESPY Awards.)

Although Jenner transitioned long after her athletic career, the (slowly) growing acceptance of those who identify as transgender means there are countless people out there who are transitioning while competing in a specific sport. New headlines come up every week—there's the South Dakota lawmaker who has proposed a visual examination of athletes' genitals; the California initiative to ban trans people from using their chosen locker rooms; the Ohio ruling that trans female athletes in high school must be checked to see if they demonstrate a physical advantage in terms of bone structure and muscle mass. Even for those most sensitive and supportive of LGBT causes, it's tough to figure out if there's a "fair" way to allow someone to play for a team that's the opposite gender from what they were assigned at birth—particularly in the case of trans women, who identify as female but presumably have (and retain) the strength, agility, body mass, and endurance of a male.

Of course, the experience of being a trans athlete is much more complex than just changing your hair and then watching the trophies roll in. The actual science behind hormone therapy or even gender reassignment surgeries doesn't provide easy answer, either—but neither medical step changes athletic ability in the way that some might think.

How a Trans Body ChangesSavannah Burton, 40, is a trans woman who plays professional dodgeball. She competed in world championship this summer with the women's team—but played for the male team before she started her transition.

"I've played sports most of my life. As a kid, I tried everything: hockey, downhill skiing, but baseball is what I focused on most," she says. "Baseball was my first love." She played for nearly twenty years—albeit as a male. Then came running, cycling, and dodgeball in 2007, a fairly new sport outside the grade-school gym. She was several years into her dodgeball career when she decided to take medical steps to transition in her mid-thirties.

"I was still playing dodgeball when I started taking the testosterone blockers and estrogen," Burton recalls. She felt subtle changes within the first few months. "I could definitely see that my throw wasn't as hard as it was. I couldn't play the same way. I couldn't compete at the same level that I had."

She describes a physical transformation that was thrilling as a transgender person and terrifying as an athlete. "My mechanics of playing didn't change," she says of her agility and coordination. "But my muscle strength decreased significantly. I can't throw as hard." The difference was especially striking in dodgeball, where the goal is to throw hard and fast at your human targets. When Burton played with men, the balls would bounce so hard off people's chests that they would make a big noise. "Now, a lot of people are catching those balls," she says. "So it's kind of frustrating that way." Throw like a girl, indeed.

Burton's experience is typical of male-to-female (MTF) transitions, says Robert S. Beil, M.D., of Montefiore Medical Group. "Losing testosterone means losing strength and having less athletic agility," he explains. "We don't know if testosterone has a direct effect on muscle strength, but without the testosterone, they are maintained at a lower pace." This means that women typically need to work harder for longer to maintain muscle mass, whereas men see results more quickly.

Beil adds that men have a higher average blood count rate, and transitioning can "cause the red blood cell counts to go down, because the amount of red blood cells and red blood cell production is influenced by testosterone." Your red blood cells are integral in carrying oxygen from the lungs to your tissues; people who get blood transfusions often feel a surge of strength and vitality, whereas people with anemia feel weak. This could explain why Burton also reported a decrease in stamina and endurance, particularly when going for a morning run.

Fat redistributes as well, giving trans women breasts and a slightly fleshier, curvier shape. Alexandria Gutierrez, 28, is a trans woman who founded a personal-training company, TRANSnFIT, that specializes in coaching the transgender community. She spent her twenties working hard to lose weight after she hit a peak of 220 pounds, but she saw all that effort literally softening before her eyes when she began taking estrogen two years ago. "It was definitely scary," she remembers. "A few years a go I used to use 35-pound weights for reps. Today, I struggle to lift a 20-pound dumbbell." It took a year of work to get back to the numbers she had pulled before her transition.

It's a fitness cliche that women are afraid to lift because they don't want bulging muscles, but Gutierrez reassures the ladies that it's really hard to get there. "I could go lift heavy weights, and my muscles aren't going to change," she says. "In fact, I actively tried to bulk up, as an experiment, and it didn't work."

The reverse transition of female to male (FTM) receives less of the athletic focus, but it's worth noting that, yes, trans men do typically feel the opposite effects, though a bit sooner because testosterone is so potent. "It can take years to develop the body you want under normal circumstances, but testosterone makes it happen very quickly," Beil explains. "It changes your strength and speed and ability to respond to exercise." Yep, it's pretty awesome to be male when you're aiming for great biceps and six-pack abs.

What's the Big Deal?Whether male to female or vice versa, a trans person's bone structure is unlikely to change in a significant way. If you were born female, you're still more likely to be shorter, smaller, and have less dense bones after transition; if you're born male, you're more likely to be taller, bigger, and have denser bones. And therein lies the controversy.

"A FTM trans person will end up somewhat disadvantaged because they have a smaller frame," Beil says. "But MTF trans people tend to be bigger, and may have certain strengths from before they started using estrogen."

It's these particular advantages that are raising tough questions for athletic organizations around the the world. "I think for high school or local athletic organizations, it's a small enough difference that people should largely ignore it," he says. "It's a harder question when you're talking about elite athletes."

But some athletes themselves argue that there really isn't an advantage. "A trans girl is not stronger than any other girls," Gutierrez elaborates. "It's a matter of education. This is totally cultural." Trans*Athlete, an online resource, keeps track of the current policies toward trans athletes at different levels throughout the country. The International Olympic Committee, for one, has declared that transgender athletes may compete for the gendered team they identify with, provided they've completed external genital surgeries and legally changed their gender.

"The science behind [transitioning] is that there is no advantage for athletes. That's one of the biggest problems I have with the IOC guidelines," Burton insists. Yes, technically trans athletes are allowed to compete in the Olympics. But by requiring a genital surgery first, the IOC has made their own declaration of what it means to be transgender; it doesn't take into account that some trans people never get genital surgery—because they can't afford it, couldn't recover from it, or simply don't want to. "A lot of people feel that that's very transphobic," says Burton.

Though both women both lost some of their athletic skill, they say the positives of transitioning far outweigh the negatives.

"I was willing to give up everything to transition, even it kills me," Burton says. "It was the only option for me. I felt like, it would be great if I could play sports after this, but it was a bonus. The fact that I'm able to play after transition is just amazing."